The present invention relates to a liquid crystal display device used for a personal computer or a work station, particularly to a technique to be effectively applied to a video signal line driving circuit (drain driver) of a liquid crystal display enabling multilevel gradation display.
An active-matrix liquid crystal display having an active element (e.g., thin film transistor) for each pixel and switching the active element is widely used as a display of a notebook computer. Because in the active-matrix liquid crystal display a video signal voltage (gradation voltage) is applied to a pixel electrode through an active element, there is no crosstalk between pixels, it is unnecessary to use a special driving method for preventing crosstalk unlike a passive-matrix liquid crystal display, and multilevel gradation display is possible.
A TFT liquid crystal display module provided with a TFT (Thin Film Transistor) liquid crystal display panel (TFT-LCD), a drain driver provided on the upper side of the liquid crystal display panel, and a gate driver and an interface section provided on the side of the liquid crystal display panel is known as one of active-matrix liquid crystal displays.
The TFT liquid crystal display module has a multilevel gradation voltage generation circuit in the drain driver and moreover has a gradation voltage selection circuit for selecting a gradation voltage corresponding to display data out of multilevel gradation voltages generated by the multilevel gradation voltage generation circuit in order to realize multilevel gradation display. This technique is disclosed in Japanese Patent Application No. 86668/1996.
In general, the gradation voltage selection circuit in the drain driver has a transistor group for selecting gradation voltages of multilevel gradation generated by the multilevel gradation voltage generation circuit in the drain driver, whose gate electrode is provided with a transistor group to whose electrodes the bit values of display data are applied.
For example, the gradation voltage generation circuit generates gradation voltages of 64 levels. Moreover, when assuming the gradation voltage generation circuit to be a decoder circuit to which bit values and their inverted bit values of 6-bit display data are input, 12 transistors are cascade-connected to the gradation voltage selection circuit for each of 64 levels of gradation.
Meanwhile, in the case of a liquid crystal display, multilevel gradation display has been further progressed from 64- level gradation display to 256-level gradation display in recent years.
Moreover, to display a picture of such a multilevel gradation on a liquid crystal display panel, the gradation voltage selection circuit in the drain drive requires 16 transistors for each of 256 levels of gradation. Therefore, there are problems that the area occupied by the gradation voltage selection circuit increases and the chip size of the semiconductor integrated circuit (IC chip) constituting the drain driver increases.
Furthermore, in the case of a liquid crystal display such as a TFT liquid crystal display module, the display screen is further increased in scale and therefore, the display screen size tends to be increased. Furthermore, to eliminate unnecessary spaces and improve the fine appearance of the display, it is demanded to minimize the region other than the display region of the display, that is, minimize the frame portion of the display (frame minimization).
However, when the area occupied by the gradation voltage selection circuit increases and the chip size of the semiconductor integrated circuit (IC chip) constituting the drain driver increases, there arises a problem that these increases cannot deal with the frame minimization.
The present invention has been made to solve the above conventional problems and its object is to provide a technique making it possible that a liquid crystal display generates gradation voltages of more gradations such as 256 gradations without increasing the chip size of video signal line driving means.
The above object and novel features of the present invention will become apparent from the description of this specification and accompanying drawings.